This invention relates to the production of wavelength-doubled light by a degenerate optical parametric oscillator (OPO).
Wavelength doubling is a potentially useful way to obtain light at a wavelength (2λ) twice as long as the wavelength (1λ) of a pump laser. For example, the Nd:YAG fundamental wavelength at λ=1064 nm can be doubled to λ=2128 using an OPO. If conversion efficiency from λ to 2λ of 30%-50% could be obtained, wavelength-doubled Nd:YAG lasers might offer a well-developed, affordable alternative to costly 2-μm lasers such as Tm:YAG or Tm:Ho:YAG. A wavelength-doubled Nd:YAG laser could then serve as the “pump” source for crystal nonlinear optics (NLO) in materials that absorb wavelengths shorter than 2 μm. For example, an optical parametric oscillator (OPO) based on the mid-IR nonlinear crystal ZGP pumped by a wavelength-doubled Nd:YAG laser might allow affordable access to wavelengths in the important 4-8 μm spectral region. The cost of such an Nd:YAG-based system may be competitive with, or even cheaper than, systems pumped by Tm:YAG of Tm:Ho:YAG lasers having comparable performance specifications.
Kalmani et al. (G. Kalmani, A. Arie, P. Blau, S. Pearl, and A. V. Smith, “Polarization-mixing optical parametric oscillator,” Optics Lett. 30 (2005) pp. 2146-2148) describes an optical parametric oscillator that uses polarization rotation in a linear retarder in combination with Type II degenerate parametric down-conversion in a periodically poled KTP (KTiOPO4) crystal pumped by a 1064-nm pulsed Nd:YAG laser to produce a single, linearly polarized beam at precisely the degenerate wavelength of 2128 nm. This approach uses a planar cavity design with intra-cavity polarization and retardation optics.
A. V. Smith and D. J. Armstrong (U.S. Pat. No. 6,775,054) report a device for optical parametric amplification utilizing four mirrors oriented in a nonplanar configuration where the optical plane formed by two of the mirrors is orthogonal to the optical plane formed by the other two mirrors and with the ratio of lengths of the laser beam paths approximately constant regardless of the scale of the device. A conversion efficiency of greater than 45% is reported.